<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Scroll Demo</title>
</head>

<body>
    <style>
        .box {
            height: 5000px;
            text-align: center;
        }
    </style>

    <div class="box" id="box">打开控制台查看</div>

    <!-- 引入节流方法 -->


    <script>
        function addListenerPos() {
            // 滚动方向枚举值
            const DIRECTION_ENUM = {
                DOWN: "down",
                UP: "up",
            };
            // 记录前一个滚动位置
            let beforeScrollTop = 0;
            function handleScroll() {
                // 距顶部
                var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
                // 可视区高度
                var clientHeight = document.documentElement.clientHeight || document.body.clientHeight;
                // 滚动条总高度
                var scrollHeight = document.documentElement.scrollHeight || document.body.scrollHeight;
                // 确定滚动方向
                let direction = DIRECTION_ENUM.DOWN;
                if (beforeScrollTop > scrollTop) {
                    direction = DIRECTION_ENUM.UP;
                }
                // 通过滚动方向判断是触底还是触顶
                if (direction == DIRECTION_ENUM.DOWN) {
                    // 滚动触底
                    if (scrollTop + clientHeight + 40 >= scrollHeight) {
                        console.log("滚动触底");
                    }
                } else {
                    // 滚动到顶部
                    if (scrollTop - 40 <= 0) {
                        console.log("滚动到顶部");
                    }
                }
                beforeScrollTop = scrollTop;
                // 打印数值
                console.table([{
                    label: "可视区高度",
                    value: clientHeight,
                },
                {
                    label: "滚动条总高度",
                    value: scrollHeight,
                },
                {
                    label: "距顶部",
                    value: scrollTop,
                },
                {
                    label: "距底部",
                    value: scrollHeight - (scrollTop + clientHeight),
                },
                ]);
            }
            function thorttle(fn, time) {
                window.flag = null;
                return function () {
                    if (!window.flag) {
                        window.flag = true;
                        fn();
                        setTimeout(() => {
                            window.flag = false;
                        }, time)
                    }
                }
            }
            // 滚动节流
            const throttleHandleScroll = thorttle(handleScroll, 100)
            // 监听滚动
            window.addEventListener('scroll', throttleHandleScroll);
        }
        addListenerPos()

    </script>
</body>

</html>